In the field of graphic arts, a printing plate is produced using a set of color separation films of a color original which are prepared using lithographic films. In general, color proofs are prepared from color separation films in order to inspect for errors in the color separation step and to check the need for color correction and the like before the main printing (practical printing operation). Color proofs are required to realize high resolution enabling accurate half tone reproduction, high processing stability and so on. To obtain color proofs close to actual prints, it is desirable for the materials of color proofs to be the same as those used on press, for example, the same paper as the base and the same pigments as colorants. There is a higher demand for a dry process involving no developing solution for the preparation of color proofs.
With the recent spread of computerized systems in prepress work, recording systems for preparing color proofs directly from digital signals (dry process) have been developed. Such computerized systems, particularly contemplated for preparing high quality color proofs, are generally capable of reproducing dot images at 150 lines or more per inch. In order to obtain high quality proofs from digital signals, a laser beam is used as a recording head, which is capable of modulation according to digital signals and focusing into a small spot diameter. Hence it is demanded to develop image forming elements that exhibit high recording sensitivity to laser light and high resolution enabling reproduction of highly precise dot images.
Recording materials known useful in laser transfer methods include a heat melt transfer sheet, which comprises a substrate, a light-heat conversion layer capable of absorbing laser light to generate heat, and an image forming layer having a pigment dispersed in a heat fusible component (e.g., a wax or a binder) in the order described, as disclosed in JP-A-5-58045. In the image formation method using such recording materials, a laser-irradiated area of the light-heat conversion layer generates heat to melt the image forming layer corresponding to the area, and the molten part of the image forming layer is transferred to the image receiving sheet laminated on the transfer sheet, thereby forming a transfer image on the image receiving sheet.
JP-A-6-219052 discloses a heat transfer sheet comprising a substrate, a light-heat conversion layer containing a light-heat converting substance, a highly thin heat release layer (0.03 to 0.3 μm), and an image forming layer containing a colorant. In the case of this heat transfer sheet, the heat release layer reduces its bonding strength between the image forming layer and the light-heat conversion layer upon being irradiated with laser light. As a result, a high precision transfer image is formed on an image receiving sheet laminated on the heat transfer sheet to form. The above-described image formation method with the use of a heat transfer sheet utilizes a phenomenon so-called “ablation”. That is, a laser-irradiated area of the heat release layer partly decomposes and vaporizes, resulting in reduction of the strength bonding the image forming layer and the light-heat conversion layer in that area. As a result, the image forming layer of that area is transferred to the image receiving sheet having the image receiving layer laminated thereon.
These imaging formation methods are advantageous in that use can be made of printing paper having an image receiving layer (adhesive layer) as an image receiving sheet material, that a multicolor image can easily be obtained by successively transferring images of different colors onto the same image receiving sheet, and so on. The method utilizing ablation is particularly advantageous for ease of forming a highly precise image and is useful to prepare color proofs (DDCPs: direct digital color proofs) or precise mask images.
With the spread of DPT work, printing companies adopting a computer-to-plate (CTP) system have a strong demand for a DDCP system, which eliminates the need of intermediate film or plate output as has been involved in traditional analog proofing. In recent years, DDCPs with higher qualities, higher stability, and larger sizes have been demanded as good approximations to the final prints.
Laser heat transfer systems, whereby images at high resolution can be formed formation, include (1) a laser sublimation system, (2) a laser ablation system, (3) a laser melt system, etc., though each of which has the problem that the recorded dot shape is not sharp enough. In the laser sublimation system (1), dyes are used as colorants, which results in such problems as insufficient final print approximation and blurred dot outlines due to dye sublimation, thereby failing to achieve sufficiently high-resolution. In the laser ablation system, on the other hand, pigments are used as colorants and thus a satisfactory final print approximation can be achieved. However, the dots are also blurred and only insufficient resolution can be obtained similarly to the dye sublimation system because of the involvement of colorant scattering. The laser melt system (3) also fails to create clear dot outlines because the molten colorant flows.
In image recording systems using laser light, use has been recently made of laser light comprising multibeam, i.e., a plurality of laser beams to shorten the recording time. When an image is recorded using multibeam laser light, however, it is sometimes observed that the transferred image formed on an image receiving sheet has an insufficient image density. A particularly remarkable decrease in the image density arises in the case of recording high-energy laser. As the results of discussions by the inventor, it is found out that the decrease in the image density is caused by uneven transfer occurring in high-energy laser irradiation.
The image receiving layer of the image receiving sheet contains a matting agent to ensure vacuum contact to the heat transfer sheet. Thus, the clearance is controlled to prevent transfer errors such as white image spots and dot defects caused by unevenness on the recording drum or dust or debris. However, a liquid coating composition containing the matting agent undergoes sedimentation with the passage of time, which results in unevenness in the performance of the image receiving sheet. As a result, there arises a problem that the transfer errors such as white image spots and dot defects cannot be sufficiently prevented.
There are additional problems such that the transfer properties onto wood-free paper (paper with high surface roughness) still remains insufficient and that the image surface is sticky after transferred onto printing paper.
There is an additional problem that so-called “picking” occurs by image defects or poor transfer release due to dust or debris in transfer onto printing paper.
Furthermore, there is a problem that only an insufficient register accuracy is achieved, thus causing image distortion.